African American women manifest a more aggressive form of breast cancer and have the highest mortality rate among women of Caucasian, Asian and Hispanic descent. According to a 2007 projected statistical report from the American Cancer Society, 5,830 African American women are expected to die from breast cancer alone(1-3). This ethnic bias towards the development and progression of breast cancer in African American women is not fully understood. Recent epidemiological studies have implicated genetic variations in the Vitamin D receptor (VDR) gene as the causative factors behind breast cancer initiation and progression. Specifically, case studies have linked the Fok1 VDR allelic variant in enhancing the risk of breast cancer in African American Women. Furthermore, a retrospective study revealed that this polymorphism together with another variant, the long or short Poly (A) was associated with increased risk for the disease(7). However, these findings have not been validated with experimental data. The major goal of this proposal is to elucidate the significance of VDR polymorphisms in breast cancer. To accomplish this, the following specific aims are proposed. Specific Aim 1: A. Establish expression constructs containing Fok1 allele specific VDR cDNAs with long (L) or short (s) Poly (A). B. Generate transfectants of these expression constructs in VDR-, ER-, ER+ human breast carcinoma cell lines. C. Determine the activity and function of the altered VDR gene by Dual Luciferase assay. Specific Aim 2: Identify the molecular mechanisms by which different VDR genotypes in the absence or presence of estrogen receptors modify(a) cell growth and proliferation (b) cell differentiation and in vitro invasiveness (c) apoptosis (d) VDR and ER mediated signaling in response to a less toxic Vitamin D analog, 1a(OH)D5. Specific Aim 3: Characterize the growth patterns of the transfected cells and determine their responsiveness to 1a(OH)D5 in athymic mice in a xenograft model. Relevance: The proposed study will help identify the significance of VDR polymorphism in breast cancer risk and responsiveness to Vitamin D. Importantly, the results obtained from this study may pave the way for understanding the possible role of VDR polymorphism in increased risk of breast cancer.